CRM addicted

The requirement I've been asked to automatically associate the account manager of a customer to an opportunity, if it takes place in a branch. In our case customers are always contacts and the main account manager of the client is a lookup field. The linking happens via a n:n relationship between the opportunity entity and a custom employee entity (schemanname = tt_mitarbeiter). The corresponding subgrid showing the participants of type employee is shown in an own section on the opportunity form. Options I know there are third party tools available, like the 365 Workflow Tools that are able to fullfill this requirement. But since I am working in a banking environment, I prefer to write the necessary code on my own. So I decided to go for a custom workflow activity, which first checks if the account manager is already associated to the opportunity, or not. If the account manager isn't added so far, he/she is associated to the current opportunity. C...