Adding custom module pages to the Solr index
Apache Solr Search Integration is a replacement for the core content search.
This means that the output of your custom modules will appear in the Solr index, if the modules create nodes.

If the content returned by the custom modules is dynamic, then they need to create dummy nodes that they will be updated with hook_nodeapi(). When ApacheSolr rebuilds its index, it loads the node with node_load(), which then invokes any implementation of hook_nodeapi(), passing "load" as operation parameter.