An Add-on is a SyncroSim Package that extends the functionality of an existing Base Package (like the stsim package). You can also develop your own new Base Package that contains ST-Sim. There are subtle differences between Add-On and Base Packages that probably aren’t important to you at this stage. Either way you would be developing a SyncroSim Package. For any SyncroSim Package you would start by developing a script in R using the rsyncrosim R package to do your spatial regression (as a pre-processing step to running ST-Sim). Once you have the script working you then simply add an XML configuration file to define how this script ties into SyncroSim. Note that you don’t need to develop a Package in order to do your spatial regression – rather you can develop a script in R that first does the spatial regression and then runs ST-Sim and never bother going on to turn this into a Package. We do this kind of thing all the time. Much like in other programming environments (e.g. R) you would only bother developing a Package if you wanted to distribute your script to others in a user-friendly way.
Unfortunately we haven’t had time yet to publish a developer’s guide for SyncroSim – we will be working on this over the coming months. In the meantime you can get an idea of how SyncroSim Packages are developed by looking at the source code for several existing packages at github.com/ApexRMS.