I am extending an aluminium ladder by bolting the lower ends to 3" wide, 1" thick struts. I am concerned that the (6mm) bolts may split the wood when loaded.
As a Structural Engineer in Training I advise to add the extension to the top of the ladder so there is no axial load through it, only shear and moment. I would recommend a minimum of a 2" thick piece so sister two 1" thick boards together nailing them every 6" together. I would advise that you match the length attached to the ladder with the length of the extension with a minimum of 12" overlap on the ladder. Add two bolts on each end with a 2" (50mm) spacing between each bolt and the edge of the ladder. Also make sure to match the rung distance if your extension is that long. Hope it works out for you!