Hi Henrique,
as far as I know there's no way to have single column values show up in a picker. A very rough solution: Why not simpy add an HTML message above/below the picker that tells "Use CTRL+Click for Multi Selection"?
This is something I have been doing for Slicers that often require you to select multiple ones. As this is similar to how MS Office works hardly anyone ever complained.
Also if you have some kind of grouping, you can add another filter with that one. After the service group (or similar) is selected the service slicer will automatically limit the selections.