Is it possible (advisable?) to make a sealed tube by melting and twisting the end of a borosilicate test tube? One issue that I can think of is that the pressure might be too high if I then proceed to heat the tube. I thought that I could maybe connect the tube to a vacuum tube and heat it in the middle. Any experience with this?
Usual way to work with air sensitive compounds is to work on Schlenk-line (search Wiki). When you transfer your substance you can leave it in Schlenk tube, or if it's really sensitive and you need to keep out of the line for longer period of time you have in general three possibilities: (1) use glassware with J. Young valve (available even for NMR tubes); (2) store your standard Schlenk glass in glove box filled with inert gas and (3) the oldest is to melt your glass.
Now back to your question. Usual way is to prepare glass with narrow place on it prior to doing experiment (that's were later you will melt it). By narrow I mean the hole inside should be ~2-3 mm (thinner glass, smaller hole). I used to use Schlenk lines, so on opened end I have standard 14 female cone that was connected to a vacuum line. I used to set vacuum ~0.1 Torr and with propane/butane/oxygen torch one can easily close it. The huge advantage of this old way is that you can prepare your glass even with 3mm thick walls that allows you to work with substances with very low boiling points (in this case to close the glass one need to freeze the stuff with liquid N2 or if not available at least cool it with dry ice bath). Of course later opening is quite tricky and usually also had to be done after freezing of chemicals.
Last part if it's advisable. That one is as usually hard to answer. I have no idea how experienced you are. Judging by question not to much (sorry). So I wouldn't start with anything to reactive in the beginning. Generally try to play first with glass and torch and get a bit used to it. Do it on proper chemical glass (not just bottle glass). If you are "advanced guy" and you want to make yourself f.e. PfBF2 borane you have to use this technique - any other gives just a miserable yield.
Possible? Doubtless. Advisable? Depends what you're sealing, I'm sure. The pressure won't be a problem though, if anything it'll be a partial vacuum inside and you'd have to heat it near the tube's melting point to get it back to atmospheric pressure. Consider that as you're heating the tube to seal it, you'll heat a good portion of the air inside as well, and once that cools it will be well below 1atm, only getting back there after heating to the melting point, the temperature at which it was sealed in.
It is indeed common, though you don't use standard borosilicate glass as I understand it. Also, it is a technique that needs special training to do right; it can be dangerous to both you and your compound if you don't get said training first. Ask around your local inorganic chem department, I'm sure someone can teach you to do it.