You can't do this with a workflow rule because a field update doesn't give you access to the existing value.
You could do this with a process, though. Create a new process that runs when a record is created/updated.
Choose Opp Product as the base object.
Set your criteria for when the opp product should trigger an update.
In that actions, choose to update a record and select the 'Opportunity Id'. That allows you to update the parent opportunity.
Choose the field you want to modify and set the type to 'Formula'. Then you use a formula to concatenate the existing value with the new value.
The formula would look like this if you wanted to add the line item description from the opp product to the description field on the opportunity:
[OpportunityLineItem].Opportunity.Description + " " + [OpportunityLineItem].Description
The process runs once per opp product that meets your criteria, so every opp product that's added or edited to meet your conditions will add to your result field on the opportunity.
Make sure your criteria only allows this to run once per opp product or you'll get duplicate values every time the product is edited.