How to create a custom keyboard

I think you're looking for the "Text, Web, and Editing Programming Guide for iOS"

The UIKit framework includes support for custom input views and input accessory views. Your application can substitute its own input view for the system keyboard when users edit text or other forms of data in a view. For example, an application could use a custom input view to enter characters from a runic alphabet. You may also attach an input accessory view to the system keyboard or to a custom input view; this accessory view runs along the top of the main input view and can contain, for example, controls that affect the text in some way or labels that display some information about the text.

To get this feature if your application is using UITextView and UITextField objects for text editing, simply assign custom views to the inputView and inputAccessoryView properties. Those custom views are shown when the text object becomes first responder...

This might serve as a good introduction: customizing the iOS keyboard

You can use Custom-iOS-Keyboards library on Github.

First of all create a view (I did it in a separate nib file and loaded it this way):

    NSArray *views = [[NSBundle mainBundle] loadNibNamed:@"ReducedNumericKeyboardView"
                                                   owner:self
                                                 options:nil];
    keyBView = (ReducedNumericKeyboardView*)[views objectAtIndex:0];

After it, I set it as input view for the text field where i want to use it (and actually, this is the short answer to your question ;) ):

    [self.propertyEditor setInputView:keyBView];  

When clicking into the field i do scroll the view pup (if necessary) to not cover the field:

CGRect textFieldRect = [self.tableViewController.view.window convertRect:propertyEditor.bounds fromView:propertyEditor];
CGRect viewRect = [self.tableViewController.view.window convertRect:self.tableViewController.view.bounds fromView:self.tableViewController.view];
CGFloat midLine = textFieldRect.origin.y+.5*textFieldRect.size.height;

CGFloat numerator = midLine - viewRect.origin.y - MINIMUM_SCROLL_FRACTION*viewRect.size.height;
CGFloat denominator = (MAXIMUM_SCROLL_FRACTION - MINIMUM_SCROLL_FRACTION)*viewRect.size.height;
CGFloat heightFraction = MIN(1, MAX(0, numerator/denominator));

animateDistance = floor(PORTRAIT_USER_INPUT_VIEW_HEIGHT*heightFraction);

CGRect viewFrame = self.tableViewController.view.frame;
viewFrame.origin.y -= animateDistance;
[UIView beginAnimations:nil context:NULL];
[UIView setAnimationBeginsFromCurrentState:YES];
[UIView setAnimationDuration:USER_INPUT_ANIMATION_DURATION];
[self.tableViewController.view setFrame:viewFrame];
[UIView commitAnimations];

When editing is finished, I do scroll the view down:

        CGRect viewFrame = self.tableViewController.view.frame;
        viewFrame.origin.y += animateDistance;
        [UIView beginAnimations:nil context:NULL];
        [UIView setAnimationBeginsFromCurrentState:YES];
        [UIView setAnimationDuration:USER_INPUT_ANIMATION_DURATION];
        [self.tableViewController.view setFrame:viewFrame];
        [UIView commitAnimations];

The constraints I use are set as follows:

static const CGFloat USER_INPUT_ANIMATION_DURATION = 0.3;
static const CGFloat PORTRAIT_USER_INPUT_VIEW_HEIGHT = 180;

static const CGFloat MINIMUM_SCROLL_FRACTION = 0.1;
static const CGFloat MAXIMUM_SCROLL_FRACTION = 0.2;

You want to set the value for

inputView

on your UITextField. You will need to fully implement a new keyboard or input mechanism in the view you provide.

Alternately,

inputAccessoryView 

can be used to add a small amount of functionality. The view will be placed above the system keyboard and will arrive and be dismissed with it.