R/3 Contact person to CRM

Guys,

The contact person number in our R/3 46C system is different than the contact person number in CRM, i.e. 4567 (in R/3) is 500009876 (in CRM). How I can determine its corresponding number using ABAP based on the R/3 contact person #? Is this data kept in some tables?

thanks,

Yan.