In times of prolonged strife, like the Nanbokuchō and Sengoku, yes. Some of the most illustrious names of the Sengoku started their careers as rōnin mercenaries.

Keep in mind there was yet no formal rule for determining classes and the line between samurai and commoners didn't exist. You can pick up a weapon, distinguish yourself on the battlefield, and get rewarded with a stipend. Then again this I think also applied to Europe.

Individuals and bands responding to call to arms expect to be compensated. While this is true everywhere, there's no restriction on just packing your bags and leave if you feel the reward isn't accurate or even joining the other side if there's a better offer (which happened way too often). From this came an elaborate system of "battle self-reports" in which individuals or band leader submit a report of basically the cost incurred: food, arms and armor, men, time, wound sustained, casualties taken, or even just "my brave actions" (which sometimes isn't really brave at all). Lords actually had people examine these reports, make sure they weren't lying, and handing out compensations. There were times when people showed up, got their "showing up" reward, and promptly left, hence the need for checking (though such desertion continued). A lot of these men were what we would call rōnin. The Toyotomi side at the Siege of Osaka was made up of almost entirely rōnin mercenaries, including most of the de-facto commanders. But both the way of showing up and the report for compensation were used by everyone.

We also hear of illustrious mercenary bands like the Tsuda clan's Negoro band and Suzuki clan's Saika band (both were famous arquebusiers and were the ones who gave Nobunaga trouble when he tried to assault Ishiyama Honganji), Murakami clan's pirate band, Hattori's (supposedly ninja) Iga band, etc.

Unlike mercenaries in the classical period (maybe?) and mercenaries today, but probably not unlike knights in Medieval Europe, Japanese mercenaries did not only fight for pay. Of course pay was very important (as was pillaging raping and looting). But distinguishing yourself on the battlefield might net you a notice of commendation from someone high ranking. From this you might get officially hired with a regular stipend, where you might be able to keep working upwards and become daimyo like Tōdō Takatora. Or at the very least you could take it as proof of your work and deeds to sell yourself higher next time, again, like Tōdō Takatora.