How to adapt a 100 mA DC ammeter to measure up to 5A?
Put the meter on a low voltage scale with a shunt resistor accross it.  That's what ammeters do internally anyway.  Most "ammeters" are sensitive voltmeters with calibrated shunt resistors in parallel.

Let's say you can tolerate 100 mV drop to measure the current.  100mV / 5A = 20 m&Omega;.  That would be the value of the shunt resistor to provide 0-5 A full scale on a 0-100 mV voltmeter range.

You also need to consider dissipation.  5A * 100mV = 500 mW.  That's certainly doable without much trouble, but you can't just plunk down a 0805 and be done with it.  Several smaller resistors might be the way to go both to use more commonly available parts and to get better power dissipation.  For example five 100 m&Omega; resistors in parallel will give you 20 m&Omega; at 5x the total power dissipation of the individual resistors.