Challenge of connecting 2 subnet

I have an existing LAN running at C class subnet with 2 win2k server. Both running with active directory and DNS. There is one linux firewall connected to internet which is serving internet services to this network.

I have now setup another network running on another ADSL router. This is basically a wireless router with 4 LAN port. It’s main usage is to serve people with wireless connection.

My main question is how do I make it possible for those with wireless to connect to the main domain? What additional hardware do I need?

I thought of 2 methods but both have some limitation. Please share any idea you may have.