workflows modify different object

Hi,
I have a case, can't handle by myself. I need the workflow which starts when modified activity object saved, check the presence of opportunity object
[<OpportunityName>] IS NOT NULL AND [<ActivitySubType>]=LookupValue("COD_TODO_SUBTYPE", "Negotiations")

and then I need to change the salesStage in opportunity object.

I suppose that I need to use the Field Update action and tamper with sht like:
JoinFieldValue('<Opportunity>',[<OpportunityId>],'<SalesStage>')="Negotiations"

but need some assistance:)
Thanks,
Luke

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as I found in other post the situation can't be resolved that way, workflows can't modify different objects. Solution is web service made for integration event generated by the workflow.