If I understand correctly, you bought a board made for a PIC but, because you are AVR programmer, you would like to stick an ATmega instead - you want an adapter for that.
Well, an image and a link would help a lot. Generally, you can use (via wires) the oscillator connections (if the quartz frequency fits for an ATmega), and voltage and ground. For other peripherals (see, is hard to make an analysis without seeing the board) you must find out what peripherals are (SPI, I2C/TWI, Onewire, etc.) and connect them according to AVR specifications... no big deal...
For LED's, relays, buttons, you decide which AVR pins must be used, because you are the designer of your application. The application which come for PIC microcontroller is not good for AVR. [Guessing... guessing...]
I'm doing exactly opposite: I'm using the peripherals of an ATmega board (EvB4.3) with a PIC microcontroller.
I'm new to AVR's but I'm making progress: I'm using an ATmega644p with Sanguino bootloader and Arduino language on EvB4.3 board:https://sites.google.com/site/funlw65/evb-projects
Again, I'm multiplatform developer.