You can copy & paste an array of samples and Guesstimate will sample from that cluster. For instance, try pasting the following into the value field of a cell: [1,1,1,2,2,2,2,3,3,3,3,3,4,4,4,4,4,7,7,7,7]

You can use tools like distshaper6 to generate arbitrary distributions, then copy the samples into Guesstimate.

http://smpro.ca/pjs/distshaper/

Guesstimate doesn't yet support an input format for distributions outside of via samples.