While reading the guide for upgrading stand alone search heads to a cluster, I noticed that you cannot add an existing search head.
It must be a new instance, or cleaned using splunk clean all,
Because our one instance had many custom scripts and settings, I don't want to wipe and upgrade this cluster yet, could this existing instance be used alongside (but separate from) the search head cluster?
You should be able to keep the standalone SH a standalone SH and search off the existing Index cluster. Just point the searchhead at the cluster. We have a standalone SH apart from our SHC for apps like DBConnect and the Service Now app.