In some PSI pen kits, some of the parts can be separated so that you can press them in separately and THEN thread the other part on or into the part you just pressed ...
Sometimes they come as separate pieces, and you just have to press the little brass fitting into the barrel (Trimline has a finial that requires this). Other times they come together and you COULD try to press them together but they recommend you disassemble it and press the fitting first, then thread the part into it. Some of the twist transmissions from certain PSI pen kits worked this way, but I don't know which ones.
In any case, that pen looks lovely ... which style kit is it?