If you already purchased the ticket then no.

Only way at this moment is to cancel the existing ticket and do ticketing all over again this time including all segments, including Japan domestic segments. This will likely increase ticket price tremendously, due to last minute and one way. Also if the ticket you have already purchased is discounted non-refundable ticket then you will lose all remaining value of that ticket.

Once you purchased two separate tickets, then there is no way to put together those two tickets as one.

Also, when trying to purchase under one ticket, specifically on your trip your international segment is on Singapore Airlines so most likely Singapore Airlines will be ticket issuing airline. If you want everything under one ticket then Japan domestic segments will be limited to fare type Singapore Airlines have access to. Sometime it will be cheaper to buy international and Japan domestic segment separately. Because often ticket issuing airline, in this case Singapore Airlines, does not have access to all discounted fares on domestic segment on ANA or JAL.

But as explained previously, having two separate tickets do have disadvantage. Bottom line, you cannot have it all. If discount fare is important then may have to get ticket separately and live with disadvantage of having two separate ticket. If want assurance and convenience of having one ticket, then may have to pay more because ticket issuing airline may not have access to all discount domestic fare in Japan. Also, discount PER-SIN-NRT fare may have a restriction that cannot add any further segments, under such situation and want Japan domestic segment under one ticket then you have to purchase more expensive PER-SIN-NRT fare which will allow adding Japan domestic segment under one ticket.

All those fun games you have to play on airline fare rules.