If the fork cartridges are anything like the Ohlins cartridges in my race bike, you can do it with a wrench without needing the special Ohlins tool. Back the preload setting completely out. Pull the inner damping rod up (with the locknut already in place), push the top of the spring down enough to slip a wrench over the damping rod braced against the top nut (top of spring only needs to be pushed down maybe 10mm in my case). Then you install the top cap and snug the locknut and fork cap in place.
The only thing the Ohlins special tool buys you is the ability to walk away from the half-finished job with the fork spring compressor in place and the fork cap not installed yet ...
If your fork cartridges require the springs to be compressed significantly more, with impractically large force, then you need that tool.