Hi, No matter what you do to patch it, it won't work unless you fix the condition that is leading to the rot. It sounds like there might be a water issue involved here where the rain is getting into the wood. What is above this column? Is it some kind of entry area roof? You might need to check the roof and flashing condition above to make sure the water is being lead away. Maybe the drip edge is inadequate. Another common column problem is one of venting. THe correct way of installing most wood columns (if they are hollow inside) is to have them resting on blocks on the bottom to let air in and allow for them to vent out the top, where there should be more venting. This venting from top to bottom allows the wood to stay nice and dry. Also, all too typically there is no venting in these small entry roofs. This can cause all kinds of rot. If your columns are hollow and tney don't vent from bottom to top, you may want to drill a hole top and bottom and insert a screened push-in soffit type button vent.
Assuming that you have fixed the problem, let the wood get completely dry, removed ALL of the existing rot, now you can begin. If the hole is more than 1" across, you need to first fix it with a "dutchman" (this is probably related to the dutch boy putting his finger in the dike). This means that you should mechanically fix the damage. This means cutting a wood patch that can be epoxied (2 part exterior like resorsanol)and/or nailed etc. in place. THis doesn't have to fit PERFECTLY, just put it in solidly and get it close. Then, you can finish the repair with some two part polyester type putty like bondo or minwax (bondo is cheaper). The trick with this stuff is to shave it to shape with a razor (the disposable "widget" works great) while it is still somewhat soft. Small batches and don't put too much hardener or you will be in for it. You never want to leave this stuff too high, because it is like iron when it is hard and is almost impossible to sand. Don't bother painting it unless the wood is 100% dry. Sand off as much paint as possible and use a high grade exterior primer like kiltz or bins (it may be best to use the laquer based if they still sell it). Then a high quality ext. oil paint and don't let a single crack of blister etc. exist in the paint. If the paint isn't kept perfect, then it will be more likely to rot again. Good luck!