...could I carry some accessories to push the 105mm 2.5/1.8 into service as a macro when needed? What would be the best routes to squeeze at least 1:1 out of it?
If you want 1:1, then you need to consider a dedicated macro lens. There are plenty of good choices in the 90-105mm range from Tamron, Tokina, Sigma and of course, Nikon.
On the other hand, if you simply want to extend the focus range of your 105 f2.5 and f1.8 lenses a bit further for casual closeups up to about 1:4 (but not real "macro"), then good quality closeup lenses as mentioned above would do the trick. These are very convenient as they simply screw on the front, and can give good quality.
Short extension tube like the PK-11 or PK-12 will also work, depending on how much magnification you want. By itself these lenses get to about 1:7.7. With the PK-11 (8mm extension) you get from about 1:13 - 1:4.8, and with the PK-12 (12mm) you get about 1:9 - 1:4. Quality is good centrally and corners improve on stopping down - often the corners don't matter much since they are not in focus anyway. Extension tubes are less convenient to put on and off, and the focus range is rather limited.
If you like the feel of your manual focus 105mm lenses and want the convenience of continuous focusing from infinity to close, then maybe one of the older manual focus macro lenses would be best, such as the Tokina 90/2.5, Tamron 90/2.8, Nikon 105/4 or 105/2.8. These only go to 1:2 but that is good enough for many subjects. If you want 1:1 the Vivitar/Lester-Dine 105/2.8 has a good reputation.