Yes, you can add more yest to a brew at almost any point if you need to. I have added extra yeast(s) to brews up to 4 weeks after filling a FV.
Things to be aware of are:
usually you will want to make sure the yeast you are adding is in date; unless it is all you have.
you will want to use a fresh unopened pack, unless you know it has been store in a ...