As the relationship between physical (bp) and genetic (cM) maps is not linear at all (across and along chromosomes), it is not possible (to my mind) to get the genetic map from the physical position of your SNP. You have to understand that the genetic map relies on the recombination rate (measured between two generations i.e. F1=>F2) that vary according to the cross you are working on (I mean, intra/inter species cross for example and its type F1, BC, F2). So you have to study the recombination pattern in a pedigree to get the genetic map. Maybe, there are already available genetic maps for your species you can base your work on. Then, there are plenty of linkage mapping software such as Crimap, JoinMap, etc... Cheers.