Can foam minoxidil be slightly heated in a microwave to convert it into a liquid?

Minoxidil  foam tends to form a liquid when it is warmed. The manufacturer has not issued any warnings regarding this issue. I'm not sure that you need to heat the foam rather than just let it sit in a warm environment. The microwave May alter the chemical formulation.. In summary, the less manipulation that you perform, the more likely that the potency of the minoxidil remains intact.PRP hair MD.com

No, I don't recommend this. I suggest seeing a hair loss expert now to go over your condition and provide you information about your best treatment options. There are great non-invasive options like prp/progesterone and/or hair transplantation. See an expert. Best, Dr. Emer

There have never been good studies on the effects of microwaving minoxidil (and I can not think of a particular reason why there likely ever will be). By choosing to microwave you are entering the unknown.Itching from minoxidil is not uncommon. For some individuals, the itching comes from irritation from an ingredient in commercial formulations of the minoxidil solution known as propylene glycol. Many, many formulations have propylene glycol. (The minoxidil foam does not contain propylene glycol). For others, the itching is from an exacerbation of underlying seborrheic dermatitis. For others there is a true allergy to minoxidil that is responsible for the itching. Anyone concerned about their itching should see a physician for guidance.
Tips for Dealing With Itching From Minoxidil 
Dealing with itching from use of minoxidil can be frustrating. A methodical approach can sometimes allow patients to solve or reduce their itching. Generally, my tips to my own patients are as follows:
1. A little bit of itching is seldom a major worry.  If it is minor or itchy events are not frequent, it may not be necessary to address the issue.
2. I often have patients with minoxidil-related itching to shampoo daily and add ketoconazole or zinc pyrithione based shampoos to their routines. Many will use these shampoos for 2 minutes in duration 3-4 times per week for 3 weeks then go down to twice weekly after that. Sometimes I may even go up to 3-5 minutes but it depends on the specific situation.
3. If the itch persists, I advise patients to try to find the Minxoxidil foam formulation (if not using already) or I write a prescription for minoxidil compounded in glycerin, water and ethanol instead. The foam formulation as mentioned lacks propylene glycol and is less likely to cause itching.
4. If still an issue and we really want to use it we may add a topical steroid in the picture. Either compounded with the minoxidil or as a shampoo once weekly.
5. Oral minoxidil 1 mg will be an option for some of my patients.
6. Going down to 2 % instead of 5% can still work well for many as is less likely to cause itching. The 5 % lotion has more propylene glycol and is therefore  more likely to cause itching.  For men, 2 % is not quite as good as the 5 % but it is a close second place. For women, the use of 2 % and 5 % are fairly equal according to all present studies that have been done.
Conclusion
All in all, there are many causes of itching following use of minoxidil. A methodical approach will frequently help solve the issue - but not always! Consultation with an expert is helpful.