Mondo would most likely not allow this, more here and here.
In short, the reason being that the current system is very simple and straightforward. Moreover the top up function would be removed once Mondo becomes a proper bank (hopefully by the end of this year).
Also some features (Pulse) of Mondo works best if there is few large topups instead of multiple smaller ones.
Adding the capability to top up any amount would also bring a ton of problems that Mondo would have to spend time to fix and make sure that it works properly. This has been discussed in depth in this forum, please have a look 