Unlike parallel resistors, the final value of which must be calculated, capacitor values in parallel are additive. If the minimal value to be used is ordered for default (in this case 50pF?), then using an additional 100pF would yield a total value of 150pF. This is assuming 100pF for the internal arm wire and cables.
I'm not sure about the physical location of additional capacitors, maybe they can be attached to load resistors. Resistors are inexpensive and in this case 100pF caps could be soldered on to a set of 62K resistors for a final value of 62K + 150pF. Otherwise, maybe they could be stuffed into the sockets along with the resistors.
I'm afraid that modifications we're asking for, including my power supply question, might of necessity drive up the price of Vista Phono 1 MKII, if implemented. After thinking about it, these considerations aren't a deal breaker for me and maybe I'll get my order in sooner rather than later. I have an old 24V DC supply from a Sota Sapphire that I could use, but it might not even be the correct voltage for DC. 12V AC might be a challenge to upgrade, but I'm not sure it needs it.