I want to add a bunch of expanded repeats in a genome fasta file, for eg. 100 ATTs at a particular location eg Chr1-1:2. How do I that and at the same time update the corresponding gtf or gff3 file bioinformatically so that the corresponding genome annotations remain accurate? I can manually add 300 to each value in the annotation file, but I believe someone will have a smarter way to resolve this.

thanks!