Government & Defence Entrance Exam

Of course, you can and you should. If you are serious about it, It's great news because If you want to crack Civil Services you need a minimum of 3 years of preparation at least. If you are going through it during your graduation it will save you plenty of time and energy after graduation. As your link is not broken with the subjects which are very useful in civil services, It is the most appropriate time to go about it. There can be two kinds of strategies you can opt for-
1. If you are very sure that UPSC or other similar administrative services are your only goal then take out the syllabus of those exams and note down major subjects. The next step will be to note those subjects you are least interested in. Now you should do your graduation with those difficult subjects and you will be able to tackle all of your subjects together without coaching because your professors will help you.
2. If you have an interesting area in any other stream go for that because mental peace is most important in this journey and you will enjoy the preparation. The most important part of both the ways is to manage your time properly, Make small goals weekly and revise timely. Best of luck I hope you will achieve everything you want.