Can I add an account to my Minecraft launcher?

Solution 1:

The launcher can remember credentials of only one account at a time but there's no problem logging out of the currently used account, then log in using a different one, then repeat the process to return to the prior one—you just need to enter the login and password every time you switch the user.

That won't exempt you from re-buying the game though—the game won't let you play normally on an account without purchase of the game registered for that account; you'll only be able to play the 'demo' mode - 5 in-game days or about 100 minutes, after which the demo session ends and you need to start a new world from scratch.

On the other hand, if you paid for it, you can download and install the game on multiple computers and play on any of them using the same account (just not two at the same time).