If you want to use .api files in FS9 you can convert them to FS9 models with ModelConverterX.
Once they are FS9 .mdl files you can then create an object library with Library Creator XML and place them in your scenery with an object placement tool such as Instant Scenery or RWY12.
Or you could simply use the .mdl in something like ADE9X.
ModelConverterX and Library Creator XML are both free and available at www.fsdeveloper.com.
You will run into issues with texture names on some objects, but they can be dealt with.
What you will find is most .api models are very simple, low poly models (which may be fine for trees) that are relics from developers who created objects for computers with sub-gig processors.
WWOD---What Would Opa Do?
Farewell, my freind (sp)