Imputation page at Wikipedia will be a nice start to understand the concept of "imputation".
From a genotyping perspective, it refers to the imputation SNPs that are not directly genotyped on your genotyping platform (for example: Illumina Human 660W-Quadv1_A) but are present on a reference panel such as the HapMap (for example: CEU, CHB or JPT populations; see HapMap tutorial).
I would like to point you to tutorials on how to use PLINK or MACH or IMPUTE for genotype imputation, these tools widely used for this type of analysis. This tutorials are not specific to your population of interest, but you can adapt them for your requirement.
Also please check the following review articles for a comprehensive overview of genotype imputation. Genotype Imputation: http://www.ncbi.nlm.nih.gov/pubmed/19706367 (PDF here) Current software for genotype imputation: http://www.ncbi.nlm.nih.gov/pubmed/19706367
Because the CHB and JPT populations are so similar genetically at points of variation, you can impute in the combined CHB+JPT. Our colleagues ran MACH for a very CEU-like population.
Added on 9 Sep 2011: I would also look at this Primer in Nature Biotech on imputation methods and software.