There’s a feature request for this: Peptide builder command · Issue #910 · OpenChemistry/avogadrolibs · GitHub
It’s also on the roadmap although I’m not sure if it will be completed for 1.99. In the meantime, I’d recommend downloading and installing the PyPept plugin (e.g., Extensions ⇒ Download) which will insert FASTA sequences.
Can you be a bit more specific or sketch what you have in mind? I’m not sure I follow either of these points.