While there are many great heat storage option commercially available, many users find it costs much less to modify a used propane tank instead. There are suppliers that provided fully modified tanks ready for install, or one can get a tank from a salvage yard and hire someone who can weld pressure vessels to do the modifications.
How to clean the tank
Often these used tanks will still have a propane smell which is the result of the chemical mercaptan that is added to propane so that it has an odor. Allowing the tank to soak with water and bleach can be helpful, you could clean it with oxyclean, and/or you could get a product like this that nuetralizes it.
There are a number of ways to pipe a tank into you heating system. Like with an unpressurized tank, maintaining stratification will give your higher temperature water near the top of the tank. This means charging the tank with hot to top and cold from bottom and drawing from the tank in reverse (hot from top and cold to bottom). For this you will need a port at or near the bottom of the tank and one near the top with tees or two ports at each location. If the tank only has ports near the top, then a dip tube could be used to direct flow to the bottom of the tank. You may want ports for temperature gauges, aquastats, temperature sensors, a drain etc. New ports can be cut and added by welders certified for pressure vessels. Pipe diameter should be 1.25 or 1.5 for good flow through the system.
Domestic Hot Water
An indirect hot water tank with a transfer coil built in can be piped in as a zone off of the heating system that can be supplied by the wood boiler or the storage tank. (a flat plate or side arm heat exchanger could be used as well). Another option would be to have a coil put into the propane tank to absorb the heat directly. This too would likely require a welder certified for pressure vessels.
You will want to be able to turn the loading pump on and off by comparing the tank temperature to the boiler temperature. A controller like this could be used to accomplish this. Another option is to build a control system like the arduino based controller I made for mine or use something like the NFCS control system.