there is no generic way to create more complex polymer architectures than chains or diamond-like networks. But the polymer and part commands are robust enough to assist you.
To create an n-block, I normally use the polymer command to create the whole chain and then use part command to change particle types of segments within each block. Same applies to bond types. If you aim at a generic coarse-grained copolymer model, one bond type between any pair of monomer types should suffice but this depends on specifications of your problem.
With the polymer command you can specify coordinates and ID of the first monomer. Therefore another option is to create one part of the chain using polymer command, then create another part of the chain with the first monomer placed next to the end of the old chain and finally use part to create a bond between the two parts.
You can of course use part command in combination with a random walk but you would need to implement the RW in TCL. A template for TCL implementation of the polymer command can be found in samples/polymer.tcl