I am not sure what you are suggesting will be faster or better that the current Iterate function.
Increasing the Geneious Prime mapper "Iterations" setting should "grow" your assembly beyond the bounds of the reference sequence, provided there are reads that overlap in a contiguous series beyond the reference bounds.
How many iterations did you try?
Try setting iterations to to a higher number, say 1000 and see if that works. Geneious will only perform as many iterations as required.
If the assembly is to slow with a high iterative value, consider using menu Sequence -> Error correct and Normalize reads to Normalize (don't error correct) your reads. This may substantially reduce the size of your data set and allow assembly to proceed faster with a high iterative setting. Once you have generated an extended consensus you can map against that with the full read set to generate a final consensus.