1] Sfas Emes - Since in the Mikdash it was taken seven days it is not Bal Tosif to take it outside the Mikdash for seven days.
2] Rav Leibel Mintzburg ztz"l: The pasuk says "ולקחתם לכם וכו' ושמחתם לפני ה' שבעת ימים" take the Lulav it on the first day and be happy all seven days. So the taking on the first day counts and extends for all seven days. So if one actually takes it the other 6 days it is not a problem because essentially it applies for the whole chag.
3] Rav L"M: The ideal form of the mitzva is to take the Lulav in the Beis Hamikdash. When one takes it outside the Beis Hamikdash it is a change of the original mitzva [חסר עיקר צורת המצוה] - not an addition. [That is why ספירת העומר in our days is not Bal Tosif if we assume that it is מדרבנן. The ideal form of the mitzva is to count after the קרבן העומר. In it's absence the whole form of the mitzva is altered. No Bal Tosif.]
4] Rav Druk: The Gemara says that the reason for the decree to take the Lulav seven days outside the Beis Hamikdash was דרישת ציון - yearning for Zion [in memory of the Beis Hamikdash]. So the mitzva is not defined as taking the Lulav seven days outside the Beis Hamikdash [which would be a Bal Tosif problem] but as being דורש ציון. No Bal Tosif on that!!
ושערי התירוצים לא ננעלו - ישמע חכם ויוסף לקח!!
