I am the GM and have been wondering if I can roll up a character be a player as well or is that just not possible?

For major NPCs I have created some as if they were Rogue Trader PCs and built them up to a certain level, so in that sense you certainly can. If you mean can you roll up a character and be playing him along with your players as if he were a PC, I strongly recommend against it. You wouldn't really be a PC because you're not a Player Character, you're the GM and you know where the adventure is supposed to go.

It's fun to play, and it's fun to GM, but at least to me when you're GMing you accept that although you're crafting the story, it's your players who are playing through it and their decisions should be what ultimately shape it. They might (and usually do) suggest things you didn't think of or try to take the story in a direction you didn't plan. If you have a character in there that's supposed to be on an equal footing with the players then that's going to be robbing them of some of their agency.

That being said, there's nothing wrong with you having allied characters with their own motivations who can compete with the players and are rolled or played exactly like them. However you should make it clear that they're either playing their own motivation separate from your players, or else they will go along with what the PCs say - occasionally interjecting important observations if the players get stuck - but should also be removable without impacting the story overmuch.