Assigning multiple participant roles in civi event, through imports
The value for all Role data is stored in a single field so I think however you tackle this you are at risk of overwriting existing Roles. Here is one process that at least makes existing Role values visible so you won't overwrite them accidentally.

Create a Profile for updating Participants that includes the Import your Contact IDs and add them to a Group

Use Adv Search to find those Contacts but set the search "Display Results As" to be Participants.

Select the records you want to update - under Actions choose Update multiple participants - select the Profile you created above - this should then give you a batch updates screen where each Role is shown as a checkbox