Hello, Sébastien.
When in ArcMap, do you see your custom solver in the drop-down for creating new layers (click on "Network Analyst" on the network analyst toolbar)? If so, when you make a new layer, do you see it appear in the NA Window (Click the network analyst window button on the toolbar to make that appear)?
If both of those are the case, you should be able to right click on your input class in the NA window and choose Load Locations. Then, be sure to load your location by geometry (select Use Geometry in the Location Position box). If you instead chose Use Network Location Fields, then you will get the network locations for the network dataset associated with the solver you loaded from.
Also, if you already loaded by location field, you can right click on your input class in the NA Window and choose Recalculate Locations. That will recalculate your NALocation fields using the geometry of the network dataset associated with your custom solver layer.
Did this help?
Out of curiosity, what does your custom solver do?
Thanks