To use this with an external temperature sensor, you'll need a thermistor with a resistance of 10K ohm @ 25C and a "B" value of 3435K which should give a resistance of ~4,16K ohm @ 50C.
If you cannot get a thermistor with the correct B value or 25C resistance, you can make a circut like this:
Rs and Rp can be calculated with the two following equations:
1/( 1/rp + 1/(rs+r25) ) = 10000 1/( 1/rp + 1/(rs+r50) ) = 4160
Where r25 and r50 is the resistance @ 25C and 50C
Note however that the thermisters resistance @ 25C must be >= 10000 and @ 50 <= 4160 for this to work
More information although slightly incorrect: http://www.users.tpg.com.au/pschamb/a7v.html