Yes putting one object next to another will do that.
I will try to describe simply a way of doing it ... using a very simple model ... but it should give you the idea
With snap to grid on....
Assuming the wall is a rectangle ... in vertex mode hi-lite the vertices that define it ... and then from top menu:
Surface---> Make hole
Now hi-lite all the vertices that define the hole :
Vertex--->Snap to grid
Vertex--->Create Convex Surface
If you view the hirarchy a new object has been created ... it will not have a name. Put your poster on this new object (Adjust the vertices of the new object AND the hole as necessary for your poster.
If the wall has a texture on it (bricks say) then texture will be messed up, and you will need to use the texture editor to remap it properly
Hope that helps
griff