Conversion of vcf to gvcf

I am trying to combine and convert two VCF 4.1 files (an SNP VCF and an INDEL VCF) to VCF 4.2 (which, if I understood correctly is the same thing as gVCF). The resulting file would then be used as input to a third-party analysis software. The files are based on the GRCh37 reference genome.

However, despite searching the forum, I was unable to find a solution that would directly combine and convert both files into gVCF. Therefore, one possibility would be to first merge the two files with CombineVariants, second, convert the resulting file to a BAM file through SimulateReadsForVariants and, third, to derive the gVCF file through HaplotypeCaller.

Would this approach work or would you rather suggest a different and maybe simpler approach?

Thanks