David
I haven't tried it. But let me ask the obvious question - - - why?
I know that adding oil to a commercial solvent based varnish results is a mixture that cures to form a slightly softer finish than straight varnish with whatever oil/solid ratio the maker has chosen. So I presume the same would be true for a friction polish.
But your question reminds me of something. There has been a lot of discussion about friction polishes in general being a mixture of an oil, a finish such as shellac, and the corresponding solvent. But there is turner out in California who has made a number of YouTube videos in which he used shop-made friction polishes consisting of a mixture of shellac and either a commercial 'food safe' bowl finish, or a commercial orange oil and wax finish.
I just looked up the commercial description of Pens Plus; it is sold as a friction polish containing walnut oil, shellac, DNA, and microcrystalline wax in some proportions.
So I think you could add more walnut oil, but I suspect the result would be to produce a solution that cures to a slightly softer finish that the original product. Or you could experiment with applying a coat of pure walnut oil, allowing it to cure, buffing slightly with either 4/0 steel wool or a gray scruffie, and then apply Pen's Plus Friction Polish as a top coat.
I use a lot of Tung Oil, and I've applied it straight from the bottle, or thinned with turpentine. Then, after the oil has cured, I've applied a friction polish over the oil as a protective finish. The Tung Oil tends to darken the wood slightly, giving it more character. One of the characteristics of walnut oil is that it doesn't darken wood nearly as much as Tung Oil, and of course, BLO is known for being even darker.
Why don't you experiment and let us know your results.