To expand on @Semen Matusovskiy's answer, here is a code snippet by @Erel on adding a button to TextField that pops up over the number's keyboard:
B4X:
Dim b As Button
b.Initialize("b", b.STYLE_SYSTEM)
b.Text = "Click me"
b.Width = 100
b.Height = 50
Dim no As NativeObject = TextField1
no.SetField("inputAccessoryView", b)
So if TextField1 has the Keyboard Type set to NUMBER_PAD in the designer (or code), then the code above will add a "Click me" button above the number pad.