Hi @Anonymous ,
I'm afraid, this can not be achieved.
When i got it right, you have a lookup on your form. This lookup in the crm points to one entry in another entity.
To add more than one entity you need an m-n relation between the entities.
In Portal, this would a kind of subgrid.
But as far as i know, there is no out of the box way to add more than one entry to a subgrid at a time.
As this is a frequent request also from our customers, i usually make a (yes dirty) workaround: i have an additional textfield which is on the form (and set to display=none) where i put a list of the ids to be linked to in. Then i have a plugin, which reads the field and relates the entities.
If somebody has a better approach, please comment 🙂
Have fun,
Christian