How can I create keystore from an existing certificate (abc.crt) and abc.key files?

The easiest is probably to create a PKCS#12 file using OpenSSL:

openssl pkcs12 -export -in abc.crt -inkey abc.key -out abc.p12

You should be able to use the resulting file directly using the PKCS12 keystore type.

If you really need to, you can convert it to JKS using keytool -importkeystore (available in keytool from Java 6):

keytool -importkeystore -srckeystore abc.p12 \
        -srcstoretype PKCS12 \
        -destkeystore abc.jks \
        -deststoretype JKS