Believe it or not, this is unlikely to be successful. Since last year's changes, agents only seem to be able to see award inventory that shows up on the website, and if you have to stitch an award together piece-by-piece, it'll be charged on a point-to-point basis.

While it's possible that a sympathetic agent might be accommodating, it's more likely that, unless the specific itinerary that OP wants is available on a SYD-LGA search, UA would charge an extra 25K miles for the IAD-LGA leg. Furthermore, unless there is additional award inventory on the partner flights, it may not be possible to add a leg to an existing itinerary at all, because the system often seems to need to revalidate availability on the existing legs. (Conceptually, it's as if UA tickets a brand-new itinerary and then cancels the old one).

BTW, my guess for the tax discrepancy would be a fuel surcharge on the SQ leg. SQ sometimes files its fuel surcharges under YR instead of YQ. UA's system will exclude YQ automatically, but it sometimes seems to miss YR. Per UA policy, no fuel surcharge is supposed to be collected on award tickets, no matter how it's coded.