Yes, I do this frequently. Here's a few rules of thumb I use to keep it sane...
Sprung from Canon
Every one of the PC's I've run as a party member was originally an NPC that was introduced as part of a module. They usually ended up having a little more face time because they had an important job such as being escorted, being an escort or holding a McGuffan of some significance. The party works with them to overcome their own objectives, during which the PC's personality and usefulness to the party is revealed.
Invited by the Party
When the fit is right, your party will extend the invitation to join them in their adventure. It's best to let this happen, and not force the hand of the DM with this. You control EVERYTHING else in the world, the ability to control whom is in the party is a privilege they are entitled to. The NPC can express great gratitude for the party's help and even pledge their sword, but no means no here.
If they choose not to take in your NPC/PC, return the NPC to his day job. If you really have a spot in your heart for him, you can plan a later module to give him more screen time. Elsewise, start rolling up your next candidate, and look forward to the session where one of your PCs is recruited permanently.
This has been mentioned many times in the above posts. I agree totally. Don't make any major decisions for the party as a PC.
This is important. When you run the PC you are NOT the DM, all DM knowledge must be compartmentalized. When your PC is being addressed, you have to answer with only the knowledge the PC possess unflinchingly. This is a developed skill, that only comes with practice and forethought while preparing for your games. This is probably one of the most challenging and truest forms of roleplaying, so good luck!
This can be an extremely rewarding experience. It allows you to play the game from the other side, and helps bolster a party with very few players. In addition, in a group that rotates DMs, it provides a vessel for you to later step in as a player.