Yes, the disc mounts are a relatively easy add to a steel frame. I wouldn't think it should cost that much. Worst case assuming you're current hub won't accept a $25 thread-on adapter:

~$250-$350 for adding the I.S. disc brake mount, all inclusive (braze & paint)?? Co-Motion will have a price for it and others could also do the work, e.g., Steve Rex who is somewhere there on the left coast.

~$130 for Avid brake caliper, 203mm rotor & proper adapter.

~$235 for a White Ind. disc hub + spokes for a rear wheel rebuild on your existing rim. Up to $375 for a Chris King.

If you KNOW that you will be a committed tandem team spending lots of time on the bike, e.g., not a first time buyer where it's not known how well riding together will work out, upgrading an existing tandem is usually not cost effective vs. a total bike upgrade, particularly when the frame doesn't fit the way you'd like or when looking at major component upgrades. $575 for a new set of cranks + $500 - $800 for a new wheelset is a big chunk of change, and all of those other things -- never mind the disc upgrade -- will nickle and dime your budget to no end (I'm still looking for the end and haven't found it yet). Moreover, tandems depreciate like everything else so for a tandem that's a few years old, the costs of the upgrades represent a significantly higher percentage of the total bike's fair market value that may or may not be recouped in the second hand market once they are hung on an older frame.