So you already have a set of bells - and want more? The new bells will need to 'fit in' with the existing set. For American bells (Schulmerich and Malmark) this won't usually be a problem as they are tuned to a high degree of consistency.
English bells, however, are tuned by ear an, particularly with older bells, may not be of a 'standard' pitch. A bell foundry will tune new bells to fit in with the existing set - but may need to borrow some others in the set during the process.
It is usually best to make sure that that new bells are cast by the same founder as the original set.