Esri has informed me that you can only create custom offline maps for collector units if you have an ArcSDE set up with your GIS server. They said ArcSDE is the only way to publish maps up to the ArcGIS Online cloud with "sync" and "offline" modes (which is required to pull the maps from the cloud to the collector unit). I am trying find any workarounds so I do not have to purchase ArcSDE.
There maybe a way with ArcGIS Server 10.3 to publish sync capable maps without SDE as they're including a "lite" geodatabase built within server 10.3. I haven't yet installed it though so cna't say for sure.
If you have an ArcGIS.com license you could always host the feature services in the AGOL and enable sync there. I did that for testing before setting up real services in our SDE.