To match constituents from your data source by searching on RE for an alias (for example if you are storing the data source ID as an alias in RE) you need to set up the way Importacular searches for matches as below:

Once you have created your constituent mapping go to the area settings and select the radio button to 'add a field to the mappings that can be used for alias search'

If you then go back into the constituent mapping you will find a new field has been added to which you can map your data source field, this will then be used as a lookup field during import.

Make sure to add a criteria set to lookup these matches too