You can but the math to work out precise nic/pg/vg% in your final mix might be a pain. Unless you're not that fussed.

I mean, you could mix them by weight.

But the heat required to get the crystals to dissolve into the liquid is going to fuck up your nicotine.

TBH, PG is very cheap as well as Citric Acid. Depending where you live you can pick it up from most wine/beer shops worth their salt, and if not both online are cheap AF. This way you can have it on hand not just for vape juice mixing but also cooking.

Like already mentioned by /u/Tamachicken this is going to be hard to figure figure out the end mix ratio, but also mentioned by /u/skivian the heat needed to make it, might not have your desired results.

The super simple solution, ;>_> would be to just buy some already done up for you.

Direct answer: Yes you can use an unflavored nicotine base to make a citric acid solution, but it's not a great idea.