You don't need to do anything special with PBP to access special function
registers. Just use the register name like you would with any other part.

You have several options;
Code:
    CAPBUF VAR WORD
    CAPBUF = 0
    
    @CAP1Buffer = CAP1BUFL
    CAP1Buffer VAR WORD EXT
    
    CAP1BUFL = 10
    CAP1BUFH = 88

Main:

WithPBP:
    CAPBUF.LowByte = CAP1BUFL    ' low byte
    CAPBUF.HighByte = CAP1BUFH   ' high byte
    GOTO Main
    
TheDarrelWay:
    CAPBUF = CAP1Buffer
    GOTO Main
    
' Or in asembler.

TheMovffWay: 
    @ MOVFF CAP1BUFL, _CAPBUF   ; low byte
    @ MOVFF CAP1BUFH, _CAPBUF+1 ; high byte
    GOTO Main

TheLongWay: ' banked method
    @ MOVLB 0x0F            ; point to bank 15
    @ MOVF CAP1BUFL,W       ; low byte
    @ MOVWF _CAPBUF,ACCESS
    @ MOVF CAP1BUFH,W       ; high byte
    @ MOVWF _CAPBUF+1,ACCESS
    GOTO Main