It’s really hard to suggest the best way to proceed without a concrete goal or example of exactly what you want to accomplish.
Part of it depends on how close your goal is to what can already be done with NengoLoihi. If it is fairly similar, then you could make your model in NengoLoihi, and then go in and just tweak the parts you need on the NxSDK board object. For example, you could build your network as normal and then go in and add delays to particular synapses (something that we don’t currently support in NengoLoihi).
One thing we’ve done before is writing custom SNIPs for a particular project. This is not too hard to do with NengoLoihi; the main thing you have to do is figure out where the neurons (or whatever else) you want to interact with are located on the board, and then you can target those things in your SNIP, and use the NxSDK interface to add the SNIP.
On the other hand, if what you want to do is very far from what can be done in NengoLoihi, then it might be better to write it separately in NxSDK. The difficulty with this is that NxSDK requires you to configure the number of chips/cores/synapses when you create the NxSDK board object; we do this here. So you’d somehow have to hack that to leave extra space for whatever you want to build, and then build into that space. One way to hack it might be to have one or more Ensembles that do nothing and aren’t connected to anything, which you can then find and overwrite on the board with your custom stuff.