Can prolotherapy help with my overall flexibility?

Yes but. Its not harmful but can be expensive and doesnt help everyone.
Increase or decrease. Are you looking to increase your flexibility or decreased your flexibility (laxity) secondary to hypermobility syndrome or "double jointed" condition? In the latter condition, prolotherapy might help strengthen ligaments which are the primary joint stabilizers. You should avoid smoking and stay within 10% of your ideal body weight. Hope this helps. www.otwithmd.com.
Not usually. Prolotherapy is designed to cause inflammation in a specific area of injury so that healing will occur. Overall flexibility would require a general improvement in metabolism. Getting a good night sleep, avoiding cigarettes and alcohol, eating 6 equal healthful meals a day, followed by relaxation and light, fun exercise would be more likely to help overall flexibility.