Is it possible to bind a msflexgrid/mshflexgrid to a custom datasource in vb6?
Yes of course you can. This was one of the new features in VB6.
See Using the DataGrid Control with a Class Module which is for a DataGrid but the same thing works with the FlexGrids.
You can create data-aware Classes and UserControls, or you can create an OLEDB Simple provider.