Signal modifier?

Is it possiable to take a variable input signal 0-1v and have the arduino modify it say .1 volts?
Ie. input .8 volts and out put .7

Sure, sort of - you read 0.8V with the analog input port, you do analogWrite with a value from 0 to 255 and pass it thru an RC filter to smooth the 0-5V square wave output into a DC looking-ish signal.
Or, add an external DAC like MCP3201 for a real DC level.