Please look at the APOE SNPedia in the following link:

http://www.snpedia.com/index.php/APOE

You can find how to convert e2/e3/e4 genotype to two SNP genotypes (rs429358, rs7412). After conversion, you can create plink files and use plink to merge your APOE plink files into ADNI-1 genotype plink files.