Partial reverse charge

we being a pvt ltd company availed man power supply from a proprietorship manpower consultancy for a month which come under partial reverse charge mechanism. in the invoice he has levied 25% of 12.36%  of total charges. so to pay remaining 75% of ST, should we register newly for this one month service receipt by amending my existing service tax registration category  or can i make the payment under the existing category as Auxillary service. if i should register what should be the category name and what is the procedure for the registration amendment.