There is a way to add, but not to remove Psycasts, and it only works outside of EdB Prepare Carefully

First of all; you'll want to enable debug mode, (usually used for testing mods)
Development mode
To use this mode, you need to activate it:
Run RimWorld;
Press the "Options" button in the main menu;
Under Gameplay, the middle column, tick "Development mode".

Then while playing, a bar should show up near the top of your screen, click the one labeled "Open Debug Actions Menu" and from there, in the "Pawns" Section of buttons (they all go down and continue into the row to the right before being separated into a new section) click "Give Ability" and from there you get a list of possible Psycasts to use, this won't work if the user isn't already a Psycaster, sadly, there's no way to remove them, as I said.

is there also a way to modify the psy focus objects?

I know it's an old post, but there is a pseudo-option for both questions.

First, lets you choose the psycasts you gain rather than getting a random one when you upgrade your psycasts: https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2293460251

Second, lets you mess with a lot of meditation-related things including the radius of the anima tree no-build zone and what meditation types are limited to: https://psteamcommunity.yuanyoumao.com/sharedfiles/filedetails/?id=2111021647