Job interviews are hard because they can ask for anything and there are barely any useful advice that would always apply. This advice does not guarantee anything. It could even backfire so take it with a grain of salt.
"I have a part time and I am looking for another part time." is one way to put it. Don't mention that income is your drive.
Walmart is so huge in terms of employees that I don't think they would have a problem with that. Many people I know that work there have a second job. If you mention that it will be obvious that the reason is income. They shouldn't ask you why a bigger stream of income is your drive. If they do, be honest, but follow it with:
"I would also like to gather as much retail experience as I can since I am pursuing a career in X or Y" or something along those lines.
If they ask you what you have to offer for the company, mention your previous retail experience. It's sort of like adapting on the spot. Like if they question why income is your drive, you tell them, but also let them know immediately thereafter, as fast as possible, that you also have something valuable to offer them. So that it won't look like you are just looking for another job. People that give the impression of 'just want a job' tend to get rejected by the interviewers because interviewers have this notion that people like that won't perform well when asked to do something. It's hard to convince them otherwise. They also think that because you have a safety net, you won't care if you get fired and therefore, you won't perform to well. Not to mention that it might bother them to have to adapt schedules. That's why it is a hard subject to approach.
If there is no spot on the conversation to insert that information, do it when they ask you about availability. Tell them that you already have another part time that you would like to keep, but you can work times with your other manager if that is the actual case. It is important to derail as much as possible that income is your drive and clearly let them know that you will work hard regardless of having another job. That your other job won't diminish your productivity nor what you can bring to them as a worker. And show them that what you can bring to them outperforms the downside of having to work a schedule. But that one is implicit when you tell them what you can bring to them. No talking about it directly. There's no need to remind them that they would have to work a schedule with you

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