Captain America uses his shield like a frisbee, and he always sticks his hand out to catch it. Would it be possible for him to accidentally catch Mjolnir instead?
It would be logical that Steve Rogers is worthy of wielding Mjolnir, but if Thor and Steve are together in battle, is it logical that Steve would be able to catch Mjolnir even when its owner is nearby?
Steve Rogers (AKA Captain America) is one of a pretty large number of people who've shown themselves to be worthy of wielding Mjolnir. We see him do so on several occasions, notably in Thor #390
Given that Mjolnir is quite happy to be held by Rogers, it's perfectly feasible that he could accidentally catch Thor's hammer during a battle. That said, I'm unaware of any instance where he's actually done so.
No. Captain America cannot simply pluck Mjolnir out of its flight path returning to Thor and simply use it because he wants to. While he has been considered "worthy" it was a "situational worthiness" subject to revocation upon the end of the emergency which allowed him to use Mjolnir in the first place.
As shown in the very common image of the Captain (Steve Rogers) using Mjolnir, he is using it because Thor is under duress. They are battling Grog and the Demons of Death. Thor is unable to summon Mjolnir to him. The Captain decides he should try and return Mjolnir to Thor, but knows normally no one can lift it. But he decides he has to try. [From Thor #390, 1988]
In this moment of extreme crisis, the Captain "levels up" in the worthiness quotient because Thor is unable to help himself and only with help will he be able to fight his way free. Steve Rogers IS worthy, but it is because in this instance he sacrifices himself as a warrior should, does he become warrior enough, and worthy enough, to wield Mjolnir and its power for even a moment. He does the right thing immediately after that and throws Mjolnir to its rightful owner.
For the record, Steve Rogers will wield the power of Thor during the 'Fear Itself' saga, but again Thor is "in extremis" and the Captain is wielding the hammer in order to bring the battle to the enemy and return the hammer to Thor. In both cases, Thor is temporarily incapacitated.
As for plucking Mjolnir out of the air or on its return flight to Thor and holding it? Not even the mighty Greek All-Father, Zeus, who is certainly nearly as powerful as Odin could not hold it after grabbing the hammer as it was thrown at him. The hammer LEFT Zeus' hands, hands capable of choking the mighty Hercules. Even with all of his physical and magical prowess, he could not overcome Mjolnir's call to return to Thor.
Has Mjolnir been intercepted and stopped by anyone? Yes. The terrifying and powerful Bor, the father of the Norse All-Father Odin caught and held Mjolnir, nullifying the worthiness enchantment and the returning enchantment. But he is Odin's father, so it stands to reason his powers would eclipse even Odin's.
That's never happened and I would say 'No', that's not how it works. If Thor threw the hammer, it would return to Thor 'mystically'. If Cap was somehow able to grab it in flight, he would be yanked off his feet and carried back to Thor rather unceremoniously. If Thor was somehow incapacitated Cap has been able to lift and wield it (being worthy) but it's Thor's mystical hammer to command. It's not a simple lump of metal, Frisbee or boomerang that can be 'picked off' or intercepted.