Hi,
I just found the answer here ;-)
http://emp.byui.edu/ZAUGGN/Calculatorfiles/calcconversions.htm
Metric Conversions on the Calculator
HP 48G
(GRN stands for the green or right arrow, PRPL stands for the purple or left arrow)
Access the UNITS menu (GRN 6). Use the menu keys (the white keys on top) to access the desired quantity. Push NXT to get more choices. Accessing a quantity with the menu key displays a list of units for that quantity. For example, pushing the menu key under LENG results in a list of units of length. The others display similar lists. We'll use length as our example here.
Converting between two units that are both listed (for example, convert 0.0715 inches to micrometers): First, get the units displayed on the menu bar as described above. Next, enter 0.0715, then push the menu key for IN, then push NXT three times until "µ" is visible, then push PRPL followed by the menu key for "µ". Presto, the answer is 1816.1 micrometers.
Converting from an unlisted unit to a listed unit (for example, convert 4.75 x 1010 picometers to centimeters): Get the unit cm displayed on the menu bar, then enter 4.75 x 1010, then GRN followed by the times button (this enters an underscore immediately after the exponent), then lower case p (alpha PRPL P), then lower case m (alpha PRPL M), then ENTER, then push PRPL followed by the menu key for centimeters. Presto, 4.75 cm. The calculator understands all the metric prefixes listed in Table 1-5 of the text, plus many others. (To get µ, push GRN STO). It properly distinguishes between mega and milli if you use M and m, respectively.
Converting from a listed unit to an unlisted unit (for example, convert 4.75 cm to picometers). The easiest way I've found is to enter 4.75_cm using the units menu, then enter 0_pm (0 GRN, then the times button, then alpha PRPL P alpha PRPL M ENTER), then push the add button.
Temperature conversions: As long as the temperatures are actual temperatures and not temperature differences, you can proceed as above. For example, in converting -35 oC to oF and K, simply access the temperature units in the units menu, then enter 35, then change the sign, then push the menu key for oC, then push PRPL and the menu key for oF (gives -31 oF), then PRPL and the menu key for K (and get 238.15 K). If, however, the temperatures represent temperature differences, such as a temperature change of -35 oC, then you have to proceed differently. See the manual for specific applications.