Well, this is definitely not the right place to discuss the 810.
As far as the 410 goes... technically speaking, the chips already have GSM built in. Of two Dragonboard 410's I have, one has an apq8016, the other an msm8916. The difference between the two chips is that on the apq, some fuse bits a popped to disable the modem. I guess that some of these boards are shipping with msm's because not enough are binning badly enough to get the apq label to satisfy demand.
However, the problem you are going to be facing, is in the board's wiring. There are simply no connections from the necessary pins of the SoC in order to actually hook up the rest of the needed external circuitry to make this work.
Your best bet will certainly end up being a USB GSM modem.