You need to be more descriptive in customary terms.
"Your own spool"? Are you making hydraulic valves from castings?
There are three spool DCV and SCV's available.
You could daisy chain three single spool valves via the PB ports if you wanted to position the controls in different locations.
You could use a three spool monoblock valve for a loader with a third function, or add remote hydraulic taps front mid or rear. or all three. !
Yes, people have done just about everything. Including tapping into the PB port.