We just started using Google Analytics on our site last week, after having previously used WebTrends for the last several years. We were wondering if there was an option to upload our IIS log files to Analytics to backload traffic data so that we can choose our own cut-over date between systems. The IIS log files are what WebTrends uses to gather its data. The Data Import options I see in Analytics don't look like this is a possible solution, but I just thought I'd ask.
Although technically possible by creating a script that would load your IIS logs and use the measurement protocol, this is not advisable. This would certainly be a significant effort and a waste of time as the value of this historical data fades away very quickly - especially if there were any other UX and feature changes on the site. Webtrends and GA are two very different tools and each one have its own way of defining metrics and dimensions.
Instead of defaulting to a technical solution that would require importing IIS logs into GA just in case there's a need, I would take a more business-oriented approach. Wait/see if there are relevant business questions that really require merging the data from Webtrends and GA. Try to see if those questions can be answered differently and then, apply an analytical approach to answer. My long experience tells me the interest for Webtrends will fade very quickly and you will find it much more interesting to work with GA and the Google suite of solutions
That being said... I guess I'm just a tad biased!!!