I found a circuit that sends data from pic dsPIC30f2012 (that pic has a resolution of 16 bits) to a DAC 12 bits (MCP4822) . My question is if I can use the SPI Module of a pic 18f2550 of 8 bits to send data to the DAC 12 bits. Can I do that or I just have to have the other PIC.
You can use any PIC microcontroller with an SPI interface with the MCP4822. Note that the SPI interface is not 12 bit... it is only the data resolution that goes to 12-bit. Read the datasheet for more info.