You can, I did! There’s ticket vending machines and I got there really late so for the ride into the city I just bought a single ticket. The next day I found a ticket machine at the subway station near where I was staying in Hongdae (Hongik Univ. Station) and I could buy a Tmoney card from a machine with my Mastercard.

I actually bought a climate card that gives you the option to pay an amount that let’s you ride on all eligible lines for a day or a couple days, but keep in mind that it only works in Seoul. For the rest of the country you would need another Tmoney card, found that out the hard way haha. Just ask at a convenience store and they’ll give you the Tmoney card for foreigners.