Opportunity: change probability on item level

Hi everybody!

I'm looking for a field in opportunity item view (BT111H_OPPT/ItemsList), where I can insert a percentage for every position. This value should show the order probability for every single position and has to be set by the user.

I found out, that there is such an attribute available at BTItemSalesSet relation. But this attribute seems to be read-only. It's set to 100% by default and I can't change this value by inserting a new one on WebUI. The only possibility to insert a new value is to change it in table CRMD_SALES.

Does anyone know, how to set a value for this attribute via WebUI or how to set this attribute to changeable?

Or is there any other approach to solve this issue by using standard attributes?

Any help is appreciated.

Regards.