Add equations that flows with the text in the keynote

One way of having equations flow with the text is to use the equation editor inside Keynote to create equations.

You can use Insert > Equation menu at the top of Keynote or press Alt ⌥+Command ⌘+E to activate the equation edtior.

The equation editor in Keynote supports both LaTeX and MathML. See this Apple Support page for more details.

Below is an example Keynote slide that includes an equation created with the equation editor:

Yes, you can. Start writing your sentence or phrase until you get to where you need to show an equation and at that position go to insert equation and type your equation. When finished entering it, click on the insert button in the equation editor and then continue typing as normal. This puts equation in-line with your text.