Hi,
I think you you have two different issues.
1: ECC and CRM contact person no. range is same.
2: Tansfer custom field from CRM to ECC.
For the first one, you have to create the contact person from one system only by keeping the no. range same in both system but in one system make it internal and in another one make it external.
for the second search SDN for transfering ECC field to custom field in CRM by vikash.krishna
Regards,
Zafar