Workflow triggers on initial creation

I have a workflow running on my custom entity that has a 1:n relation to opportunities. There is a field that needs to be calculated on the opportunity side.  For a certain change in my custom entity, i iterate all the opportunities and update the value. 

This workflow works well on my custom entity. On the initial deployment, we want to update all the values for all opportunities for my custom entity. 

Meaning, for each record of my custom entity, i want to call this workflow one time, to update all values in the opportunity? Do i have to write another workflow code for this? 

Thanks.