I just found the miracle glue for adding small parts to soft plastic figures, like bayonets or scabbards. It is very tacky so the bond is immediate and it dries flexible so it won't come off at the first contact. I also use it glue plastic tips or butt spikes to metal piano wire spears and pikes.
Bayonets can be easily made using very thin plastic strip. A diagonal cut will give it the right shape. This is your one chance at getting the length right. In my experience anything under 5mm looks too short and over 6mm too long. Some armies carried very long blades so check your references.
The Airfix WW1 Germans have pocketknives instead of the long 50cm bayonet (7mm in 1/72).
The Hat WW1 Russians come without bayonets. They are a must considering the Russians had them on permanently and carried no scabbards.
The Hat Dutch Infantry come with short bayonets and no sabre and scabbard. Converted here to militia, I have added a plastic rod scabbard. Of course there is no way that thick bayonet could fit in the scabbard...
The Hat cataphract has a 25 thou piano wire shaft and a tip that comes from the Zvezda Macedonian cavalry. For the buttspike, the piano wire was machined into a point with a Dremel tool.
The Zvezda Companion has a steel pike with plastic tip and butt.