help me understand a simple layer 3 firewall scenario

So I was thinking about this the other day and it somewhat confused me, probably because I do not truly know routing in depth, and I feel somewhat silly even asking this question but here it goes:

say you have a router and network A 10.0.1.X and network B 10.0.2.X and your firewall is set to block traffic from incoming 10.0.2.x to 10.0.1.x and your rules end in ANY ANY. Would it ever be possible for someone to put a router in (on LAN B) and add a different subnet and it would go around the deny rule as the packets would be coming from a different network? I understand if the traffic came through as nat'ed it would get blocked, but don't route protocols just automagically populate route tables. I apologies if this is a dumb question but it just had me thinking.

Thanks!