Craig Morgan and Ben Pringle have rejected new contracts at Rotherham and will leave the club on free transfers.
Saido Berahino can leave West Brom "if the deal is right", says manager Tony Pulis.
Central defender Konrad, 24, signs from SV Eintracht Trier 05 having spent the last two seasons at the club.
Tankulic, 23, joins after successful spells with FSV Mainz 05 II and VfL Wolfsburg II.
Both players enjoyed a successful period on trial with the club and join Paul Hartley’s squad ahead of the 2014/15 season.
Dundee manager Hartley said: “They impressed when we were away in Hungary and in the Manchester City game showed what they can do.
“They are both at a good age and fit into the squad we are building. They are two good additions.”
The pair joined Dundee for their pre-season training camp in Hungary at the beginning of July, and returned to Scotland with the squad to play in their 2-0 friendly victory over Manchester City on July 13.
Both players impressed against the English champions, with Tankulic scoring and Konrad producing a strong defensive display.