That's the problem, you can not use decimal fields as de category of enchancement of the structure doesn't allow it. You can search for a more detailed information about that as is a question more related to the data dictionary (netweaver) than CRM aplication group.
You can check on SAP marketplace if there's any note that supports this, but I wouldn't put all my hopes on that. I faced a similar requeriment some time ago and what I did is convert the currency into char and store the char fortmat, each time I would need to use the field I would need to convert it to decimal again, not nice. You can also try to find another spot less restrictive to add the fields.
Cheers!
Luis