In case you don’t find any ready solution here is how to do it manually, assuming that you have
then the full sequence will be (pseudocode in java)
vSeq.substring(0, vRef - 3) + cdr3Seq + jSeq.substring(jRef + 4)
Hope I haven’t messed the indices : )
The missing things are the UTR of V segment (you can try to fetch them from e.g. UCSC genome browser) and constant region. As for the constant region, its not resolved in most of the sequencing protocols.