The m7.9 x .6 would leave you 1mm total wall thickness or .5 each side for a roller ball nib to F/p adapter. Thats about .020 plus the threads. The Heritance 8.4 x.5 would leave 1.4mm or .7 each side apx .028 wall thickness plus the thread. if you drilled the adapter for a .250 hole If you used a "C or "D" drill bit you could add a couple more thousand. The Meister nib and bock nibs in number 5 size possibly could be done if you threaded where the body of the feed is for the rollerball adapter and the refill would just slide through the hole for the 6.5 threads. I will have to do some dimensioned drawings to see if there is enough wall thicknes left for the rollerball adapter to work in that scenario