Hey that's a great idea!
So Aggregate can use a local postgresql database as datastore.
At the same time, GeoServer can use a postgres/postgis table as data source.
There's more - QGis can access postgis tables too.
If the postgresql instance is accessible to both Aggregate and GeoServer (and QGIS), then one could publish the data from Aggregate trough GeoServer as WMS or WFS or analyse it directly in QGIS. In the simplest case, you can run postgresql, Aggregate and GeoServer off the same machine.
@elanioazul if you want to connect Aggregate and GeoServer/QGIS, you'll need to set up your own server and install the required software packages on it, there's probably no way around that.
For a development environment your local machine will do, but to use a setup like this in production, you'll need a dedicated server.
If I understand your last question correctly, are you looking to record location both by "tap to record current location" and (if current location is not available) a "enter location other than my current location"? The latter could be done through ODK Build > Location widget > Style "Manual (no GPS)". You may need to load mbtiles for the offline map.
Hope that helps!