usb boot flash drive with extra space on it. Can it be used?

I have a bootable usb drive with extra space on it. Do i have to create another partition to use the extra space, or can i put everything i want on the device into one folder and copy it to the drive right along with all the installation files? Would this mess up my multi-boot operation or installation if I were to add a large folder with all my other 'stuff' to it? By the way, this is for a pc and using windows files.

It depends on the partition table and file system in the USB drive. Do I understand correctly that you have a multiboot system in it? In that case I am almost sure that you can edit the partition table, either change the size of one of the partitions or add a partition. This should be done, when you have booted from another drive. You can use gparted to do it (it comes with the Ubuntu iso files).

But if you have a live-only boot drive with an ISO 9660 file system, the partition table is read-only. In this case you can create a new persistent live USB boot system into it. mkusb can create such a system with one partition for persistence, 'casper-rw', and another partition for storage and communication with Windows, 'usbdata'.

If you post the output of the following command lines, I can give better advice,

sudo parted -ls
sudo lsblk -f
sudo lsblk -m