I'm trying to record a macro that will paste stuff from another work sheet,
then update the formulas by adding the items that are in the pasted part to
the ones that were already there. Whenever I try to do it the macro stops
recording until I get out of the formula.

You've reached the threshold that separates recording macros and
writing code from scratch. From here, to update a formula in a cell,
you'll have to know how to parse strings of text- basically taking the
formula apart letter by letter, and replacing old elements with new.
Can you give an example of the old formula, and what has to be updated?