Error while saving object

Dear,

I want to add a calculated fieldto my OpportunityProductInterest object.

i add it with the Usr prefix (UsrMarge) , but when i save the object, the system return me the following error:

Unable to save structure. Name: OpportunityProductInterest. 
Label: Product of the opportunity. 
Error: InvalidNameException. 
The "Min Quantity" code in the "From" column must start with the prefix "Usr". 
Contact the system administrator.

This is because the previous developper created fields without the prefix.

Can i change the system parameters to set the prefix not required ?

Thank you,

Nicolas