Can you use an oil extractor on transmission oil?

It can be done, to a certain extent. A tube may not go all the way to the bottom of the pan, but as the dipstick has to get a good way into the reservoir you will be able to get some of the fluid. However, it's not a good idea because:

• You will only get some of the fluid. A tube is unlikely to get all the way to the bottom of the pan, so you'll only get a fraction. Dropping the pan and letting it drain out will get much more fluid out
• The tube could get stuck in a gear, it's unlikely but it would be bad
• If the fluid needs changing so does the filter, and the filter is very important in the health of the transmission
• A fluid/filter change lets you check for metal, which is important. Metal filings or shreds is a sign there's a problem, and it's cheaper to deal with these things while they are small problems