In Mieszko I's times? Very doubtful. Bohemia was a Christian state, after all, somewhat more civilized that freshly baptized Poland. Mieszko I had empress Teofano's support (or rather he supported her) which was enough to conquer Lesser Poland and Silesia (which probably were poorly defended), but to conquer Bohemia Poles would have had to cross mountains with easy to defend passes. I'm also not sure if Teofano would have agreed that any of her vassal would become so powerful.