Converting stanfit object to rstanarm object

It seems bayesplot works only with objects returned by rstanarm package. If I have stanfit object, how can I use bayesplot. For example, something like this:

ppc_dens_overlay(y = y_data, yrep = posterior_predict(, draws = 50))

posterior_predict only works with packages like rstanarm that know what the model is. The bayesplot package works fine with the MCMC draws from any Bayesian model but you have to construct the posterior predictive matrix yourself in this case.