Ok...I'm not very knowledgeable about amps and cabs and what not so I figured I'd ask you guys. Here goes:

If one has an amp...lets say a low wattage (10-5) tube combo amp and it has the ability to hook up to an extension cabinet, will said cabinet make the amp louder than amp would normally be without said extension cabinet?
No. Adding cabs doesn't add volume, unless the speakers in the cab are appreciably more sensitive (you can look up that number on the manufacturer's website, it's a dB rating at a certain frequency).

Assuming the speakers are the same, more speakers will not add volume. They'll spread the sound out better, which in some cases is can be enough to substitute for a bigger amp. With 5-10 watts, though, if you're trying to keep up with a drummer, you're probably going to need a bigger amp.
The amp in question is quite loud and easily went over the drummer in practice...how large a venue do you think I could play with an extension? also...if there was a PA system then amp could be mic'ed and therefore plenty loud enough, correct?
yepp, deoending on your genre, 10w is not enough for a band. even 60w all tube is not enough for my band, our guitarists 6505+ combo is always cranked to 10 and it barely keeps up with the volume. part of it is the number of seakers though. im playing through a 4x12 cab while he plays a 1x12 combo....