Its not difficult at all
the way i do this is to modify the 18f452.INC file in pbp to the following :
;************************************************* ***************
;* 18F452.INC *
;* *
;* By : Leonard Zerman, Jeff Schmoyer *
;* Notice : Copyright (c) 2001 microEngineering Labs, Inc. *
;* All Rights Reserved *
;* Date : 12/19/01 *
;* Version : 2.41 *
;* Notes : *
;************************************************* ***************
NOLIST
ifdef PM_USED
LIST
"Error: PM does not support this device. Use MPASM."
NOLIST
else
LIST
LIST p = 18F452, r = dec, w = -302, f = inhx32
INCLUDE "P18F452.INC" ; MPASM Header

; Configuration BYTE 1H Options
;_OSCS_ON_1H Oscillator Switch enable
;_OSCS_OFF_1H
;_LP_OSC_1H Oscillator type
;_XT_OSC_1H
;_HS_OSC_1H
;_RC_OSC_1H
;_EC_OSC_1H External Clock w/OSC2 output divide by 4
;_ECIO_OSC_1H w/OSC2 as an IO pin (RA6)
;_HSPLL_OSC_1H HS PLL
;_RCIO_OSC_1H RC w/OSC2 as an IO pin (RA6)


NOLIST
endif
LIST
EEPROM_START EQU 0F00000h
BLOCK_SIZE EQU 8





i then use the following
'@ __CONFIG _CONFIG1H, _OSCS_OFF_1H & _HSPLL_OSC_1H 'for 10mhz running at 40mhz


@ __CONFIG _CONFIG1H, _OSCS_OFF_1H & _HS_OSC_1H 'For normal 8-20mhz


Toni