Been there done that. My girlfriend has a suzy DR250 which I has a similar linkage setup to the DR400. While stripping and cleaning the linkage and swingarm on her DR and my '98 WR400 it occured to me to install grease zerks on my bike as well. (The DR's bearings were mint after 10000 km, some of mine were rooted from water damage, jet wash maybe, after 2500 km). Yamaha
have gone half way through the manufacturing process for installing zerks and maybe some of the parts are common to other models which may be fitted with zerks but, big but, the WR parts were hardened before the required additional drilling threading and turning operations were performed. After doing the aditional work I decided the whole process wasn't worth the effort and I would have been better off stripping and repacking the bearings every few months. Also, the seals and bearing collars have a different configuration compared to the DR and were not designed to cope with internal grease pressure. The seals are displaced when pumping grease in and this can't be good. I reversed some seals to allow grease to escape. My advice, don't bother. Strip and repack every now and then.