Depends on the age of the already cooked stuff. If it's been around for a long time and not been used for maybe a few weeks and gone all flaky and dry as a bone, it's best to start from new. Years ago - decades in fact, when I worked in a place that undertook lots of restoration and made traditional furniture in old mostly Georgian or thereabouts styles we almost exclusively used hide glue, and there were four or five pots of the stuff about in constant use. They were topped up as and when required at any time during the week, but we used a soft cake version of the glue rather than pearls. We simply cut a chunk off and threw it in the pot adding water when needed to keep viscosity right. Once a week, late on Friday afternoon, all the pots were emptied of glue, washed out, some fresh cake put in the pot, plus a bit of water, ready for the following Monday, or Saturday morning if we were doing overtime.
So, in answer to your question after the preamble above, start afresh if the glue in the pot is pretty ancient and nasty, but if you've got a pot that's running short of glue but has been in pretty constant use for a few days, then my experience tells me you can just go ahead and add a bit more of your softened (water soaked gloop) of glue pearls. Slainte.
