Is it safe to inject the piriformis?

The small triangular depression that occurs on either side of the nose is sometimes referred to as the pyriform space. It can deepen with age and is often an area that needs more volume for a youthful appearance.
Cautious injection into the area can be safely accomplished however I consider this an advanced injection technique because there is risk from underlying blood vessels. It is important the the injector be experienced and well versed in the anatomy of the face as well as safety precautions and treatment in the rare occurrence of a complication.

You appear to be referring to the piriform fossa, where some filler is placed to help soften nasolabial folds if this is due to a receding maxilla.
As always any injectables require an initial consultation and as far as risk factors go, the piriform fossa is one of the riskier sites to inject so while it is entirely possible to have a complication related to this, what is more important is to choose an injector who is qualified and experienced and able to recognise and manage any complications if they arise.