No, it's not possible to follow users on PieFed, only communities. The only way to get content from Mastodon is if it is posted in a community in one way or another.
There are a few active a.gup.pe groups that can be seen from PieFed, such as !knitting@a.gup.pe. Hopefully native group functionality will arrive in Mastodon at some point, making integration a bit more smooth. :)