Is there a way to make pink peppercorns properly grindable in a pepper mill?
Honestly this isn't something I've thought about before, as when ever I've used pink peppercorns they've been mixed with black, green and white ones. Which I assume help's dry out the softer and wetter pink corns stopping the grinder from clogging. 

Moving forward my first and main suggestion would be to treat it like any other spice, and use a pestle and mortar. Asides from definitely not clogging with this method, you will also have much greater control over how fine/coarse the result is. 

Another possibility would be a spice grinder though, probably no use for small amounts. 



On a side note:

Apparently pink peppercorns can affect persons who are allergic to tree nuts.

They are also deemed as a toxic by the FDA, although not at a life threatening level

https://nutgourmet.wordpress.com/2014/07/19/warning-pretty-pink-peppercorns-can-be-dangerous/