Syncing the iPod does not make podcasts that are in your iTunes library disappear. If you use automatic syncing, syncing may remove podcast episodes on your iPod, depending how the iPod's Podcasts tab is set up.
Select the iPod in iTunes. There is a row of "tabs" (starting with Summary). Click on Podcasts there. This is the Podcasts tab, where you tell iTunes how to sync podcasts to the iPod.
Sync Podcasts is checkmarked, if you have been using automatic syncing for podcasts. Below that, there is a setting that says Automatically include [option] episodes of [option]. If that checkbox is checked, and the first option is all upplayed, or any choice that says unplayed, iTunes will remove played podcast episodes when you sync the iPod. That is probably why previous podcasts were removed. You can adjust the options for this setting as desired.
If you want to pick and choose podcast episodes without iTunes doing anything automatically, just uncheck that checkbox. You can then select the specific podcast episodes you want on the iPod "manually" from the listing below. However, new podcast episodes will not be added automatically either; you'll have to select the new episodes as the appear on the list